Method and system for determining the position of a portable image detector assembly with respect to an emission point of an x-ray source in a radiographic system

ABSTRACT

A method and system for accurately localizing a wireless radiographic detector in a radiography system, based on a method to accurately determine distances between a set of generator arrays and sensor arrays. The generator arrays and sensor arrays are preferably orthogonally arranged magnetic field generators and sensors, that allow measurements of distances without being affected by presence of human tissue between the generator and sensor arrays.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2018/057856, filed Mar. 28, 2018. This application claims thebenefit of European Application No. 17165497.3, filed Apr. 7, 2017,which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention generally relates to the localization of the position of aradiographic detector in a radiography system and more specifically tomobile radiography systems, wherein a preferably wireless image detectoris positioned behind the object to be imaged, but of which the exactposition is not always accurately known. An accurate determination ofthe position of the image detector in relation to the X-ray source isimportant for determining the imaging parameters, the tube and patientpositioning.

2. Description of the Related Art

It has been a longstanding problem in conventional radiography toaccurately determine all parameters of the acquisition geometry of aparticular radiography system applied to a particular exposure setup. Inconventional radiography, the beam of an X-ray source (in most cases anX-ray tube) is modified or shaped to optimally expose the patient tissuesubject to examination to render an optimal image quality result, whileminimizing the patient exposure to radiation. The part of the X-ray beamexposing the region of interest of a patient is partially attenuated bythe tissue encountered by the beam on its path to the imaging detector,and forms a latent image that is accumulated (CR-image) or integrated(DR-image) in a detector sensitive to X-rays. So-called exposuresettings fully determine the quality of the X-ray beam and have animportant impact on the resulting image. The exposure settings areessentially determined by the type of exam to be carried out, and dependon the following: type of examination (soft tissue examination requiredifferent settings than the examination of bony structures), age of thepatient (pediatric or not), specific acquisition geometry SDD (sourcedetector distance), ODD (object detector distance) and incidence angleof the beam) and thickness of the patient. The exposure settings aredefined in terms of mA (electric current passed through the anode of theX-ray tube), s (exposure duration in seconds), and kV (tube voltage inkilo-volts).

In fixed radiography installations, the acquisition geometry isrelatively easy to be determined since the degrees of freedom of themovements of the different modality components are defined by themodality design and the movements of the individual components can beeasily tracked by measuring their displacements. The exact locations inspace of the main determining components for the acquisition geometry(namely the X-ray source, the image detector and table surface) can berelatively easily calculated by tracking or measuring the movements ofthose components relative to reference positions. Digital readings ofthe displacements or rotations of a C-arm gantry can unambiguouslydefine the emission point and inclination angle of the X-ray beam, forinstance. In a typical fixed radiography installation, also the locationof the image detector can be unambiguously located because of the factthat it resides in a so-called detector “bucky” of which the location inthe table is predetermined or can be easily measured.

The situation is different for older radiography equipment that don'thave digital position readings on all the components, and also formobile radiography devices or for examination types which require thedetector to be outside the bucky. Mobile X-ray devices are used as aversatile solution for acquiring radiography images under circumstanceswhere a patient cannot be easily transported to a dedicated X-ray room,or cannot be positioned easily. Mobile X-ray devices are used inemergency rooms, in interventional setups, or in cases where the patientneeds an examination in his hospital bed. It is clear that under thesecircumstances it is a lot less obvious to accurately determine theacquisition geometry when the relative position information between thedifferent components, such as the X-ray source and the image detector,is not available.

Especially, the determination of the exact position and orientation ofan image detector relative to the X-ray source is challenging since thetwo objects typically do not have a mechanical relationship, and mayeven not be in a line of sight of each other. One has to assume that theobject or patient to be imaged obscures the image detector when lookingfrom the source perspective. Therefore, distance-measuring techniquesthat require a clear line of sight cannot be applied. In the art,different distance measuring techniques relying on a variety of effectshave been proposed as a solution. Some of the solutions rely partly orentirely on the integration of kinetic sensors, which can record andcalculate displacements based on the acceleration measurements, orangulations with respect to—for instance—the gravity force. Thesesolutions require complex calculations of the recorded movements and arenot sufficiently accurate for the application described here as thesensor may drift over time. A solution based on this principle alsoalways requires a calibration step, into which the object to be trackedhas to be registered in a reference position. Additionally, the type ofkinetic sensors used in this kind of application have certain intrinsiclimitations that do not allow certain parameters to be measured. Forinstance, acceleration sensors are not able to detect a rotation of anobject oriented in plane parallel with the ground surface (perpendicularto the force of gravity). As an example, US patent US2014376700 (SamsungElectronics Co. Ltd.) proposes a solution to align at least theorientations of the X-ray tube and the detector using such anglemeasurement sensors.

In the same disclosure, also magnetic field sensors are proposed tomeasure and detect a relative position of an X-ray image detector (towhich they are attached) relative to a magnetic field generator which isconnected to an X-ray source assembly. The magnetic field generatorgenerates a static magnetic field, of which the intensity is measured atthe positions of the magnetic field sensors and brought in relation withthe relative distances between the magnetic field sensors and themagnetic field generator (the latter is positioned close to the X-raysource). The magnetic field is approximately inversely proportional tothe third power of the respective distance between the magnetic fieldgenerator and the individual magnetic sensors. The measured magneticfield intensity values are thus indicative of the distances between themagnetic field generator and the magnetic sensor. The magnetic fieldmeasurements are used in combination with the acceleration sensors inorder to obtain a relative location and relative orientation of an imagedetector, which allow, in combination with the determination of a startlocation, to calculate absolute distances from the X-ray sourceposition. This solution is however not very accurate, and is verysusceptible to external influences which may influence the magneticfield such as the presence of metallic objects or the presence of otherstatic or non-static magnetic fields, such as the earth's magnetic fieldor the presence of electromagnets or coils in the vicinity of thedetectors.

U.S. Pat. No. 9,179,886 (Carestream Health, Inc.) discloses an approachwhereby a method for alignment of an image detector with the beam of anX-ray source is also based on magnetic principles. In the disclosure, atime-varying magnetic field pattern is generated at a position connectedto the X-ray source, so that this signal can be picked up and measuredby a number of magnetic sensors (at least 2) which are connected to theimaging detector. The magnetic field pattern is generated at apre-determined frequency chosen so that the signal is made transparentfor human tissue (i.e. that the energy is not absorbed by human tissue).The advantage of the alternating magnetic field is that the amplitudemeasurements allow to compensate for any present static magnetic fieldssuch as the earth's magnetic field. The time-varying signal is picked upand measured at multiple spatially distributed magnetic sensor elements(or coils) which are arranged at fixed positions of an image detector,and of which the combined read-outs induced by the generated magneticfield are indicative for the location of the detector in relation to themagnetic field generator. The technique is thus an improvement over thepreviously mentioned solution in that it is less susceptible to externalmagnetic disturbances, but at the same time, it is clear that thetechnique does not intend (and does not achieve) to obtain absolutelocation measurements of the sensors attached to the detector. Thedisclosure achieves a more reliable estimation of the relative positionsof all detectors in relation to the magnetic field generator. Bycomparing the measured values against a set of reference values, themethod is capable of providing an indication of the alignment of theimage detector with the X-ray source location.

An important aspect that contributes to the accuracy of the system isthat the different magnetic sensors are differently aligned, althoughonly in the same plane as the image detector. At least two magneticsensors are required, and are preferably aligned under at least 45° ofeach other. Additional magnetic sensors may be added to further improvethe accuracy of the method. Additional detectors are preferably arrangedin the same plane but under different offset angles.

In another disclosure (U.S. Pat. No. 7,581,885) the idea of using a setof 3 GPS sensors built into the corners of an encasement of an imagingdetector is used to perform the absolute GPS-localization of the 3sensors (measurements which rely on triangulation techniques fordetermining the individual position of the GPS sensors), using thesatellites of the GPS. In the document, the idea is raised that once theexact locations of the 3 sensors in the corners of the imaging detectorare known, also the relative position of the imaging detector withrespect to the X-ray source can be calculated. Today, it is howevergenerally known that the accuracy of the (civil) GPS is insufficient forthis intended application of locating an object on the sub-centimeterlevel, as the standard horizontal accuracy of a civil GPS receiver underoptimal signal receiving conditions is 3.5 meters. The most optimizedGPS location enhancement techniques (such as RKP used in the miningindustry) result in an accuracy of 4 cm. Especially when it is generallyknown that many external factors can degrade the GPS positioningaccuracy (such as signal blockage in buildings, or signal reflectionsagainst walls), it is clear that the accuracy of the proposed systemwill especially suffer under the operating conditions of such an X-rayimaging detector alignment system; namely inside a hospital building.

In summary, a number of solutions have been described in the art whichat best resolve one of the partial problems encountered when looking outfor a full and accurate solution to determine the position andorientation of an image sensor with respect to the position of an X-raysource. Many good and accurate solutions exist in case that directline-of-sight distance measurement techniques can be used, but onlypartial solutions exist when the visual path between the source and thedetector is blocked. The most promising techniques evading thedirect-line-of-sight limitation are based on magnetic field measurementtechniques, but so far, only estimations or relative measurements withlimited accuracy and reliability have been achieved.

SUMMARY OF THE INVENTION

The invention provides a method for determining the relative position ofa portable image detector assembly with respect to an X-ray sourceconfiguration in a radiographic system, the method comprising the stepsof generating a time dependent sequence of orthogonally arrangedalternating magnetic fields in at least 3 spatially distributedgenerator arrays which are connected to said X-ray source, measuring thesignals induced by said sequence of (9) alternating magnetic fields inat least 3 detector sets each consisting of an orthogonally arrangedmagneto meter triplet, said detector sets being spatially associatedwith said portable image detector assembly, calculating (9) absolutedistances from each generator array to each detector set based on fieldstrength information only, and obtain coordinate data for each detectorset by performing trilateration on said (9) absolute distances.

It is an object of this invention to resolve the above mentionedproblems, and provide a system which allows accurate and reliablepositioning of an image detector with respect to the position andorientation of an X-ray source in a radiography system.

In order to resolve the longstanding problem in radiography ofidentifying in a reliable way the location and orientation of an imagingdetector with respect to the position and orientation of an X-ray beam,it is necessary to be able to find a technique which allows to determineall 6 degrees of freedom that can determine said position andorientation. While other techniques, referred to above, are limited to 4or 5 degrees of freedom (because they rely on partially relativemeasurement techniques such as angle measurements or alike), ourinvention resolves this limitation by determining the actual spatialcoordinates (x, y, z) of three known points in the imaging detector.These three known points refer to the physical locations of threesensors of which the exact location is determined by the method of thisinvention. These three points mathematically define a plane in which theimage detector is oriented. But also they define the rotation angle ofthe image detector about all axes, and the distances from the referencepoint located on the X-ray source assembly. Knowing the threecoordinates for the three sensors determines the 6 degrees of freedom ofthe position of the imaging detector in space relative to a referencecoordinate system determined by the location of three signal generatorsattached to the X-ray source assembly.

The accurate determination of the image detector with respect to theorientation of the X-ray beam, allows then different practicalapplications in radiography, such as an automated and accuratedetermination of the SID (source image distance) which is an importantparameter to select the exposure settings for any type of radiographystudy. Orientation and position data allow automated or guidedadjustment of the alignment of the detector (and patient) with the X-raybeam. Another application can be found in the automatic determination ofthe acquisition geometry of a radiography system in a tomosynthesiscontext, wherein an accurate knowledge of the acquisition geometry isnecessary for accurate image reconstruction.

The actual determination of the spatial coordinates (x, y, z) of thethree signal sensors in the imaging detector is based on a knowncalculation technique used in the field of GPS; namely trilateration.The method relies on reliable measurements of the distances between atleast three signal generators (the satellites in GPS context) and asignal sensor. In the assumption that the exact location of the threesignal generators is known, then also the exact location of the signalsensor registering the at least three signals from said three signalgenerators can be derived through this triangulation calculation. Thekey to the calculation are the reliable measurements of said distances.

In the context of this invention, the measured signals are indicative ofthe absolute distance between a signal generator and a signal sensor,which means that every measured signal is directly related to anabsolute distance (in a linear or non-linear fashion). Differentmeasurement techniques or physical principles may be envisaged allowingdirect derivation of an absolute distance from a measured signal.Examples may be for instance, the measurement of an acousticecho-signal, a radar signal, a laser distance measurement, magneticfield strength, or alike.

In the context of this invention, a signal generator consisting of atriplet of orthogonally arranged coils is called a generator array.Similarly, a signal sensor consisting of a triplet of orthogonallyarranged coils is called a sensor array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a mobile X-ray device [100]to which a set of 3 generator arrays [111] is attached by means of astructure that is fixed to the X-ray source assembly or collimator[110]. The 3 generator arrays are fixed in a frame-like structure whichkeeps the generator arrays at a fixed distance from each other, and ofwhich the dimensions are known. The image detector assembly [120] isshown in conjunction with the X-ray device. A signal [122] is sent fromone of the generator arrays, and is detected by all 3 sensor arrayswhich are integrated into or attached to the image detector assembly[120].

FIG. 2 shows a details of the X-ray source assembly to which the set of3 generator arrays [111] is attached. The detector arrays are spacedfrom each other. The arrow [130] represents the SID (source to imagedetector distance) and can be obtained through measurements performed bythe method of the invention. The SID is the distance between the centerof the image detector assembly [120] and the emission point [112] of theX-ray source.

FIG. 3 shows the concept of trilateration which is also applied in GPStechnology, which allows to accurately calculate the coordinates of aGPS-receiver (represented by one of the cars in the picture) under theminimum condition 3 satellites send a signal that allows theGPS-receiver to determine its distance to all of the 3 satellites. Theminimum requirement to apply this technique is to have at least 3satellites providing the distance information. In our invention, theGPS-satellites are replaced with generator arrays that each provide aconsecutive signal to each of the sensor arrays (in analogy with thecars in the figure) that can be interpreted by the sensors as theirrespective distance to the generator array. In the invention, all sensorarrays (cars) calculate their distances to the 3 generator arrays(satellites), after which 3 trilateration calculations are executed toobtain the coordinates from the sensor arrays (cars): (x1,y1,z1),(x2,y2,z2) and (x3,y3,z3).

FIG. 4 shows an embodiment of an X-ray detector assembly [120] which on3 of its corners is foreseen with a sensor array [121]. In thisconfiguration, it is obvious that the internal distances between thesensor arrays [120 a], [120 b] and [120 c] are constant and known, andtherefore can be used to verify the accuracy of the obtained sensorarray coordinates via the method of this invention.

FIG. 5 shows an embodiment of a collimator [110] aligned with a set ofgenerator arrays [111] which are associating their configuration inspace with the alignment of the X-ray beam (represented by the Z-axisindicated in the figure).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of this invention is based on the use of a set of atleast three signal generators, which are physically connected to anX-ray source assembly of a radiography system. The at least three signalgenerators are mechanically fixed to the X-ray source assembly in orderto fix a reference coordinate system in space which is selected as thepoint from where the X-ray radiation originates and that determines theorientation of the X-ray beam. This virtual point is called the emissionpoint of the X-ray source. At least three signal generators are requiredto determine the location and orientation of the reference coordinatesystem. Adding more signal generator elements may contribute to a betteraccuracy of the system, but is strictly speaking not required. The atleast three signal generators are so arranged that they are spatiallyseparated from each other (for instance located at positions around theX-ray source assembly) so to ensure that their generated signals appearto be different for detectors at different positions.

In a preferred embodiment, three signal generators sequentially orsuccessively generate their signal, meaning that the generators arenever operated simultaneously, but one after the other. The sequentialor successive signal sequence is then repeated until the detectorposition measurements are completed.

The particular aspect or characteristic of the generated signal thatdetermines the absolute distance between the signal sensor location andthe generator location is then picked up by signal sensors withindetection range of the signal generators. A prerequisite for a signalgeneration and sensing technology to be used in this application is thatat least one measureable parameter of the signal (such as the intensity,the amplitude or signal strength) should be dependent on the physicaldistance between the signal generator and the signal sensor itself. Itis conceivable and acceptable that additional mathematical operationsand calibrations are required to convert the signal data into a distancevalue. Preferably, the measurable parameter of the signal should not beeasily influenced or degraded by environmental factors, and should betransparent for human tissue.

Further to our preferred embodiment above, there are three signalsensors built into the image detector assembly, of which the position isbeing detected. At least three of the above mentioned signal sensors arerequired for the invention to work, and they are required to be as faras possible spatially separated from each other to obtain the mostaccurate results. An obvious design choice for the location of the threesignal sensors in an image detector assembly would be in three of fourcorners of the assembly frame. Since the determination of the exactpositions of the three signal sensors is the purpose of the method ofthis invention, it is clear that these three positions can uniquelyidentify the location and orientation of the image detector assemblywhen the dimensions of the image sensor assembly and the locations ofthe signal sensors within the assembly are known. In geometry, twopoints identify a line in space, three points define a plane. Theinvention therefore requires at least three signal sensors, whichcompletely define the position and orientation of the image sensorassembly, while every additional sensor may only contribute to anincreased accuracy of the calculation. Moreover in the case that theinternal distances between the signal sensors (or the dimensions of theimage detector assembly) are known, the accuracy of the measured signalsensor locations of the three signal sensors can be verified bycomparing the known internal distances of the sensors against themeasured internal distances resulting from the measured locations of thesignal sensors.

The three signal generators produce a continuously cycling time-basedgenerator signal sequence so that all possible distance measurements(consisting of three simultaneous measurements between the active signalgenerator and the three signal sensors) can be performed one afteranother. As explained above, the location of the signal sensordetermines the measured signal value which is picked up by each signalsensor for every generator signal. This results in our particularembodiment in a total of nine signal measurements. These measured valuesare then converted into distance measurements, and fed into atrilateration algorithm which calculates the 3D-coordinates of thepositions of the signal sensors relative to the position of the(virtual) emission point.

The trilateration algorithm is a known mathematical method thatessentially identifies the point in space that intersects with three(imaginary) spheres (in the case of our embodiment spheres around thethree signal generators) defined by the positions of the signalgenerators as the center of said spheres and the above mentionedcalculated distances as their respective sphere radii. In theory, theresults of this equation are 2 points in space, of which one can beeasily ruled out since the only valid one should be found in thedirection of the X-ray beam, while the other is mirrored through theemission point towards the rear of the X-ray source assembly (and thusaway from the X-ray beam direction).

As explained above, this means that in case that any additional signalgenerators are added to the system (so that there are more than threegenerators in total), the resulting distance measurements to thedifferent signal sensors will contribute to the accuracy of thecalculation of the coordinates by adding a supplementary sphere whosesurface should also mathematically intersect with the coordinate pointof the signal sensor. In case that the additional sphere does notexactly intersect with the location of the signal sensor calculated fromthe first 3 spheres, then this data can be used to interpolate betweenthe different possible solutions (or other optimization algorithms knowfrom GPS technology can be used).

A further important aspect of the invention is the appropriate choice ofdistance detection principle that can fulfill the above-mentionedrequirements of accuracy and transparency to human tissue. A promisingstarting point appears to be magnetism, as it is known that magneticfields pass through non-metallic objects. Nevertheless, other pervasivewaves, beams or radiating energies may be considered as alternatives. Itis known in the art that the human body is transparent for alternatingmagnetic fields, on condition that the frequency is below 100 kHz. Whenthe frequency raises above 5 MHz, human tissue starts to absorb aconsiderable fraction of the emitted energy and the signal becomes moresensitive for electromagnetic disturbances caused by electric circuitsin the vicinity. These elements make frequencies above 5 MHz less usablefor the purpose of distance measurement. The reason for usingalternating magnetic fields as opposed to static magnetic fields is thatthis approach compensates for the presence of any static magnetic field,such as the earth's magnetic field in case that the amplitude of thereceived signal is taken as the parameter for distance measurement.

Therefore it is contemplated to use resonant magnetic signal generator-and sensor-pairs to optimize the magnet signal transfer to perform thedistance measurements. In a preferred embodiment, the signal generatoris a coil through which an alternating electric current is sent by anelectronic generator circuit. The alternating electric current inducesan alternating magnetic field of the same frequency which can bedetected at a distance from the generator coil. A similar or smallercoil is used as the signal sensor and picks up the alternating magneticfield passing though it; inversely the alternating magnetic fieldinduces an alternating current in the sensor coil that can be picked upand measured. The detector coil may be smaller in size and have adifferent winding number in comparison with the generator coil. Thecircuit that reads out the alternating electric current in the sensorcoil is “tuned” to the frequency of the generator (hence the name“resonant”) by means of the selection of the correct capacitor andresistor network, in order to optimize the sensitivity to the generatorsignal frequency.

While in the above preferred embodiment, the magnetic generator andresonant magnetic sensors are coils, different types of magnetic sensorsmay be used as alternatives, such as MEMS, Hall effect based sensors,magneto-resistive sensors, or alike . . . .

The amplitude of the measured alternating current in the resonantmagnetic sensor has a relation with the distance between the signalgenerator and signal sensor. The amplitude is approximately inverselyproportional to the cubic distance from the signal generated, whenmeasured in radial direction from the generator. When the sensor coil iskept at the same distance to the generator but its orientation and/orthe relative orientation is changed the received signal ranges from 100%to 0%.

${L_{m} = {\frac{{const}.}{R^{3}}\left( {{3{\left( {\hat{a} \cdot \hat{r}} \right) \cdot \left( {\hat{r} \cdot \hat{b}} \right)}} - {\hat{a} \cdot \hat{b}}} \right)}};$

The mutual inductance Lm depends on three vector products; a is a vectordescribing the orientation of the generator; b describes the orientationof the sensor and r the relative orientation between sensor andgenerator.

The maximum amplitude can be measured when the vector determining themagnetic field in a certain position aligns with the sensor coil axis.In all other cases only the component of the magnetic field having thesame direction as the sensor coil axis will contribute to the measuredvalue in that particular sensor coil. Relying on the amplitudemeasurement for distance determination would only be a viable solutionin the case that no angular variation could be expected (and this is notthe case in our application).

Another aspect that influences distance measurement values based on theamplitude read-out from a single coil sensor are disturbances of themagnetic field, such as the presence of metallic objects in the vicinityof the sensor coil. Such disturbances have not so much an impact on theamplitude component of the magnetic field vector, but rather on thedirection of the magnetic field vector. This means that suchdisturbances nevertheless can have a significant impact on the measuredamplitude by the sensor coil in a certain position. This is anotherreason why relying on the amplitude measurement of a single magneticsensor alone is not sufficient to determine the distance in a reliableway between sensor and generator.

Replacing the above described single coil (or other type of magneticgenerator and sensor) structures in both the signal generator and signalsensor with a triplet of orthogonally arranged coils increases thedistance measurement accuracy drastically, but only on condition thatthey are operated in a particular way which is disclosed in thisinvention. In the context of this invention, a signal generatorconsisting of a triplet of orthogonally arranged coils is called agenerator array, and a signal sensor consisting of a triplet oforthogonally arranged coils is called a sensor array. While a singlecoil generator read out by a single coil sensor results in a singleamplitude measurement, reading out all combinations of a generatortriplet and a sensor coil triplet leads to 9 amplitude signal readouts,being 9 induced currents and resulting voltages:

$V_{m,n}^{ind} = \begin{bmatrix}v_{11} & v_{12} & v_{13} \\v_{21} & v_{22} & v_{23} \\v_{31} & v_{32} & v_{33}\end{bmatrix}$

To simplify the calculation of distance and coordinates of the sensorarray it is of advantage to square the voltage matrix elements and todefine a signal matrix S_(i,j) and the total signal S_(tot) as well asthe signal components S_(x), S_(y) & S_(z):

${S_{i,j} = \begin{bmatrix}v_{11}^{2} & v_{12}^{2} & v_{13}^{2} \\v_{21}^{2} & v_{22}^{2} & v_{23}^{2} \\v_{31}^{2} & v_{32}^{2} & v_{33}^{2}\end{bmatrix}};{S_{tot} = {\sum\limits_{i = 1}^{3}{\sum\limits_{j = 1}^{3}S_{i,j}}}};$${S_{x} = {\sum\limits_{i = 1}^{3}S_{i,1}}};{S_{y} = {\sum\limits_{i = 1}^{3}S_{i,2}}};{S_{z} = {\sum\limits_{i = 1}^{3}S_{i,3}}};$

The distance between the generator array and the sensor array R can becalculated from S_(tot) and the axis intercepts or coordinates x; y & zfrom S_(x); S_(y) & S_(z):

${R = \sqrt[6]{\frac{6 \cdot {{const}.^{2}}}{S_{tot}}}};$${x = {\frac{R^{4}}{{const}.} \cdot \sqrt{\frac{{5 \cdot S_{x}} - S_{y} - S_{z}}{18}}}};$${y = {\frac{R^{4}}{{const}.} \cdot \sqrt{\frac{{5 \cdot S_{y}} - S_{x} - S_{z}}{18}}}};$${z = {\frac{R^{4}}{{const}.} \cdot \sqrt{\frac{{5 \cdot S_{z}} - S_{x} - S_{y}}{18}}}};$

It follows from the above formulas that it is possible to directlycalculate the exact position (expressed in the coordinates x, y & z) ofa sensor array. However, this method is not desirable for directdetermination of the locations of the sensor array. The formulas aboveimmediately illustrate that the determination of each coordinate is verysensitive to changes in the respective angle between the generator anddetector coils.

In contrary to the sensitivity of the determination of the sensor coilcoordinates through measurements of the magnetic signals as describedabove, the determination of the absolute distance R is not sensitive tothe orientation of the sensor arrays with respect to the generatorarray; the distance R only depends on the summation of the signalsgenerated in the coil triplet and picked up in the 3 sensor coils. It isupon this aspect that a reliable calculation of the absolute distance isbased for further application in the trilateration method describedabove.

The distance measurement using the above mentioned set of coil tripletsrelies upon the sum of the squares of all voltage matrix elements. Thismeasurement is surprisingly stable against external fields ordisturbances induced by metal nearby as it uses all measured values(generated by all signal generators, and picked up by all signaldetectors). The robustness can be explained by the fact that thedistance is calculated from the complete magnetic field of all threeemitter coils and not only from field components. Introducing a distancedependent gain matrix can compensate deviations from the simplifiedmodel used here which assumes that the distance R is many times largerthan the size of the coils themselves, so that the coils may be assumedto behave like dipoles. The gain matrix can also correct for differentcoupling efficiencies between different receiver and generator coils.The measurement of the coordinate components x; y & z is less stable asonly one receiver coil is used for each coordinate axis. Additionallythere are different positions which produce the same voltage matrix—alllocations that are point-symmetric to the emitter (which results in anuncertainty between 2 different possible solutions giving the samevoltage matrix). But this problem is not important as for a radiographysetup only half of the sphere is used—we always work in the samehemisphere, namely in the direction of the X-ray beam.

Electrical conductors such as metal plates or objects close to emitteror receiver produce interference and thus distort the measurement. Thisis caused by so-called “eddy currents” induced in the conductor by thealternating field of the emitter coils. The eddy currents in a conductorproduce a magnetic field opposed to the inducing field. The magneticfield thus gets weaker (the eddy currents drain out energy from themagnetic field). On the other hand the eddy current also induce acurrent in the sensor coils. In case of resonate circuit design there isno phase difference between the directly induced currents and the eddycurrent induced currents. As a result, the measured (and thus observed)distance can get larger or smaller dependent on the location andorientation of the metal object. The impact on the measured distancedepends on the distance of the conductor to the signal generator arrayor sensor array respectively. The influence on the x, y & z coordinatesand the orientation of the sensor array is much larger than thedistortion of the distance measurement when using the coil tripletconfiguration. The magnetic field induced in the conductor depends onthe orientation of the conductor to the generator. Thus, the magneticfield is not reduced symmetrically but mainly in the directionperpendicular to the conductor so that the measured field strength ismainly changed for the sensor coil which is oriented almostperpendicular to the plate. In conclusion, eddy currents influence thedistance measurement moderately (a slightly larger or smaller distanceis measured) while coordinate and orientation measurements areinfluenced strongly.

Another potential source of distortion of the measurements are externalalternating magnetic fields. Almost any electrical device producesalternating electrical fields in a wide frequency range. The influenceof the fields strongly depends on their frequency and on the design ofthe read-out for the sensor coil. If the frequency of an external fieldfits the design frequency of any sensor coil the influence on themeasurement accuracy can be very large. External magnetic fields maythus influence the measurement accuracy but the influence is lesspronounced compared to eddy currents as the external field typically hasno fixed phase relation or do not fit to the design frequency.

So, in conclusion, measuring absolute distances using magnetic fieldstrength measurements can be made robust when using coil tripletconfigurations as alternating magnetic field generators and sensors forthe above-mentioned reasons. In a preferred embodiment at least three ofsuch alternating magnetic field signal generators (each consisting of anorthogonally arranged coil triplet) are used in combination with atleast three such alternating magnetic field signal sensors (each alsoconsisting of an orthogonally arranged coil triplet). As explainedabove, a reliable distance calculation can be made when for eachgenerator array; alternating magnetic fields are generated successivelyin each of the 3 coils making up the triplet. These successive signalsmay be then read out be all sensor coils simultaneously of each signalsensor triplet or sensor array. Reading out the data from all respectivesensor coil triplets results in the distance calculations between thegenerator coil triplet or generator array in question and the respectivesensor coil triplets or sensor array. When the same process is repeatedfor all generator arrays (at least 3), an accurate calculation can bemade based on the trilateration technique of the position of the imagedetector assembly in which the different sensor arrays are integrated.

It is important that the signal readout circuitry can identify thesource (which source generator array and which coil in the array) fromwhich the signal originates in order to ensure that the measured values(and thus the distances derived from it) are associated with the correctsignal generator coil (or thus distance measurement). Another aspectwith regards to synchronization between the generator and the measuredsensor values is that it is important to know exactly when a newsequence starts so that the start and end of a signal phase can beaccurately demarcated. Only when this can be achieved, can multiplemeasurement samples be taken accurately for the same assumed sequence.

In order to achieve this, various techniques may be applied such as forinstance the transmittal of a synchronization signal indicating thestart of a generator signal sequence as an electrical signal across anelectrical connection from the generator coil driver circuitry, or as anencoded time-dependent signal embedded in the generator signal that canbe detected wirelessly while reading out the signal sensor data. One ofsuch exemplary signals may include a “blank phase” in the signalgenerator sequence during which no signal is generated in any of thesignal generator coils. A predetermined threshold value transition willthen determine the start and the end of the signal data phases.

One particular embodiment of this invention uses a method wherein ablank phase (wherein no signal is generated in any of the coils) is usedas an encoded synchronization signal that marks the start of a fullmulti-coil (and even multi-triplet) generator sequence. A blank sequenceis encoded into the successive coil generator sequence whereby the sumof the squares of the different measured signal sequences is used tocalculate a reference threshold which marks the signal level below whichthe synchronization signal can be recognized. In fact, this referencethreshold is calculated at 30% of this sum of squared signal levels.Every measured signal transition from below this reference threshold toabove this reference threshold marks the start of the blanksynchronization signal. This point in time can be used then to ensurethat the read-out measurements are synchronized to the correct activesignal generator coil, which induced the measured signal value.

This reference point can be measured for each sequence or a dedicatedcalibration cycle can be started at defined time intervals tosynchronize the clocks of the sensor and generator circuits. In bothcases, the sum of the squares of all induced signals in a single sensorarray is used to determine the reference point. This can be doneindependently with each of the sensor arrays to ensure high reliability.

Moreover an additional test of the integrity of the reference point canbe performed by using the expected form of the induced signals. Criteriafor a reliable measurement (wherein n is number of generators) are thedistance between two intersections is 1/(n+1) of cycle length or3n/(3b+1) of cycle length, or alternatively the number of intersectionpoints is n+1 or n+2.

The calculations involved in the execution of the method as explained inthis invention may be carried out by means of standard computerequipment or a standard computer configuration, and may be embodied as acomputer program, or alternatively may be embodied in a dedicatedprogrammed circuit allowing to perform these calculations.

1-12. (canceled) 13: A method for determining a relative position of aportable image detector assembly with respect to an emission point of anX-ray source in a radiographic system, the method comprising the stepsof: sequentially generating a signal in at least three spatiallydistributed signal generator arrays that are spatially associated withthe emission point of the X-ray source; simultaneously measuring thesequentially generated signals with at least three signal sensor arraysthat are spatially associated with the portable image detector assembly,the measured signals being indicative of absolute distances between thesignal generator arrays and the signal sensor arrays, respectively; andobtaining coordinate data for a position of each of the signal sensorarrays by performing trilateration on the absolute distances between thesignal generator arrays and the signal sensor arrays, respectively. 14:The method according to claim 13, wherein each of the signal generatorarrays sequentially generates a sequence of orthogonally orientedalternating magnetic fields; each of the signal sensor arrays measuresan orthogonal signal component v_(i,j) of a magnetic field induced byeach orthogonally oriented alternating magnetic field generated by thesignal generator arrays; and a calculation of a distance Ri between oneof the signal generator arrays and one of the signal sensor arrays isbased on the measured values of all orthogonal signal components v_(i,j)measured by the one of the signal sensor arrays and generated by the oneof the signal generator arrays. 15: The method according to claim 14,wherein the distance Ri between the one of the signal generator arraysand the one of the signal sensor arrays is calculated as:${R_{i} = \sqrt[m]{\frac{{const}.}{S_{tot}}}};{wherein}$ m = 6;${S_{i,j} = \begin{bmatrix}v_{11}^{2} & v_{12}^{2} & v_{13}^{2} \\v_{21}^{2} & v_{22}^{2} & v_{23}^{2} \\v_{31}^{2} & v_{32}^{2} & v_{33}^{2}\end{bmatrix}};{S_{tot} = {\sum\limits_{i = 1}^{3}{\sum\limits_{j = 1}^{3}S_{i,j}}}};$${S_{x} = {\sum\limits_{i = 1}^{3}S_{i,1}}};{S_{y} = {\sum\limits_{i = 1}^{3}S_{i,2}}};{S_{z} = {\sum\limits_{i = 1}^{3}S_{i,3}}};$${V_{m,n}^{ind} = \begin{bmatrix}v_{11} & v_{12} & v_{13} \\v_{21} & v_{22} & v_{23} \\v_{31} & v_{32} & v_{33}\end{bmatrix}};$ and v_(i,j) represents the measured values of allorthogonal signal components in the one of the signal sensor arrays. 16:The method according to claim 14, wherein a frequency of each of theorthogonally oriented alternating magnetic fields is below 100 kHz. 17:The method according to claim 15, wherein a frequency of each of theorthogonally oriented alternating magnetic fields is below 100 kHz. 18:The method according to claim 14, wherein the signal generator arraysinclude configurations of orthogonally arranged coils. 19: The methodaccording to claim 15, wherein the signal generator arrays includeconfigurations of orthogonally arranged coils. 20: The method accordingto claim 14, wherein the signal sensor arrays include coils, Hall-effectsensors, magneto-resistive sensors, MEMS, or magnetic field sensors. 21:The method according to claim 15, wherein the signal sensor arraysinclude coils, Hall-effect sensors, magneto-resistive sensors, MEMS, ormagnetic field sensors. 22: The method according to claim 13, whereinthe positions of the signal sensor arrays are swapped with positions ofthe signal generator arrays, and the positions of the signal generatorarrays are swapped with the positions of the signal sensor arrays. 23:The method according to claim 13, wherein each of the signal generatorarrays sequentially generates a sequence of orthogonally orientedalternating magnetic fields, and a signal generator array of the signalgenerator arrays is identified as actively generating a signal byincluding an encoded signal in any of the orthogonally orientedalternating magnetic field sequences. 24: The method according to claim14, wherein a signal generator array of the signal generator arrays isidentified as actively generating a signal by including an encodedsignal in any of the orthogonally oriented alternating magnetic fieldsequences. 25: The method according to claim 15, wherein a signalgenerator array of the signal generator arrays is identified as activelygenerating a signal by including an encoded signal in any of theorthogonally oriented alternating magnetic field sequences. 26: Themethod according to claim 13, wherein a quality of the positions of thesignal sensor arrays is verified by comparing all known distancesbetween the signal sensor arrays against calculated distances betweenthe signal sensor arrays from the coordinate data of each of the signalsensor arrays. 27: The method according to claim 14, wherein a qualityof the positions of the signal sensor arrays is verified by comparingall known distances between the signal sensor arrays against calculateddistances between the signal sensor arrays from the coordinate data ofeach of the signal sensor arrays. 28: The method according to claim 15,wherein a quality of the positions of the signal sensor arrays isverified by comparing all known distances between the signal sensorarrays against calculated distances between the signal sensor arraysfrom the coordinate data of each of the signal sensor arrays. 29: Amethod for determining a source to image-receptor distance bycalculating a distance between a center of a portable image detectorassembly and an emission point of an X-ray source, the methodcomprising: calculating the center of the portable image detector usingthe method of claim
 13. 30: A method for aligning a portable imagedetector assembly perpendicular to an emission point of an X-ray source,the method comprising: iteratively calculating distances between aplurality of signal sensor arrays and the emission point of the X-raysource during positioning of the portable image detector assembly suchthat all calculated distances are equal. 31: A system for determining arelative position of a portable image detector assembly with respect toan emission point of an X-ray source in a radiographic system, thesystem comprising: a set of at least three spatially distributed signalgenerator arrays that are spatially associated with the emission pointof the X-ray source, and that sequentially generate signals; a set of atleast three signal sensor arrays that are spatially associated with theportable image detector assembly, and that simultaneously measure thesequentially generated signals; a processor configured or programmed tocalculate absolute distances between each of the signal generator arraysand each of the signal sensor arrays, respectively, based on themeasured sequentially generated signals; and a processor configured orprogrammed to calculate coordinate data for a position of each of thesignal sensor arrays by performing trilateration on the absolutedistances between the signal generator arrays and signal sensor arrays,respectively.